Custom default QuantityFormat and UnitFormat

[puigru]

Currently, both Quantity#toString and Unit#toString default to a built-in formatter, SimpleQuantityFormat and SimpleUnitFormat.
I believe it would be useful to provide a way to set a custom formatter globally, especially for Quantity. Meaning any time a Quantity is converted to a String within the application it would use the provided formatter.

indriya/src/main/java/tech/units/indriya/AbstractQuantity.java

Lines 295 to 297 in cb717f7

	public String toString() {
	return SimpleQuantityFormat.getInstance().format(this);
	}

[keilw]

We did try this, check out the second ticket here #2, but it had so many unforseen side-effects on formatting and parsing, that we abandoned it. Feel free to fork the code and demonstrate it works, then we may revisit it, but given the built-in Date/Time library of the JDK has ultra-primitive toString() functionality in most of its core types like DateTime, LocalDateTime, etc. and Instant does exactly the same default formatting:

    public String toString() {
        return DateTimeFormatter.ISO_INSTANT.format(this);
    }

we do not feel a strong need in another value-based Java API to go beyond what the JDK felt was necessary.